Water flowing in natural or artificial channels exerts a force on the channel bed in the direction of flow. This force is known as the tractive stress and is dependent on the unit weight of water, depth of water, and the energy slope. This force has the ability to erode or scour an unlined channel bed. The typical equation for modeling erosion in open channel flow is expressed: EQU E.sub.r =K(T.sub.e -T.sub.c) [1]
where
E.sub.r =the erosion rate in volume of soil per unit time per unit area PA1 T.sub.3 =the local effective stress PA1 T.sub.c =the critical stress PA1 K=a constant of proportionality, (soil erodibility factor)
In high stress applications, the critical tractive stress is small in comparison to the effective stress, and the equation essentially can be expressed as: EQU E.sub.r =K(T.sub.e) [2]
The effective stress is the stress at the soil-water interface causing detachment. As the effective stress is increased in the channel, the rate of erosion is increased by a factor of K. Defining the erodibility, K, is therefore necessary in determining the erosion rate for the stresses anticipated for design conditions.
A report by the American Society of Civil engineers (ASCE) task committee on Erosion of Cohesive Materials (ASCE, 1968) drew three conclusions related to small apparatus for the purposes of determining soil resistance properties to erosion: (1) Much progress has been made on apparatus to simulate erosion forces; (2) Problems still existed in translating the results to design criteria, and; (3) Simple devices that allowed soil conditions to be easily controlled, or undisturbed samples to be tested, need to be further developed. An assessment of different types of test apparatus is discussed by Hollick, M. 1976 "Towards a routine test for the assessment of critical tractive forces of cohesive soils", Trans. of the ASAE, 19(6):1076-1081. Many of the devices have been developed to assess the critical tractive stress.